The present invention relates to a hypodermic syringe with aspiration effect, comprising a cylindrical member or ampoule, a piston rod and a piston, which is connectable or combined to an aspiration rod and arranged to cooperate with the piston rod.
At odontological local anaesthesia the intention is to locate the anasthetic in the tissue. Intravasal injection, e.g. injection into a vein, should be avoided from medical and practical point of view, as the risk for secondary effects is large and the anasthetic is rapidly washed away from the injection area. It is therefore a requirement that an aspiration shall be made before the injection. Aspiraton should always be accomplished when you want to be sure that the cannula at intravenous injection has punctured a vein or an intramuscular injection has not entered a blood vessel.
For known hypodermic syringes with aspiration effect the piston can be so designed, that a diaphragm is forced to curve inwards in the cylindrical member when a pressure is exerted on the piston rod. A small quantity of fluid is thereby pressed aside and when the pressure on the piston rod is reduced, the diaphragm returns to its original position. Hereby a negative pressure will arise in the cylindrical member and fluid from the injection area may flow into the cylindrical member. The disadvantage of such a hypodermic syringe is that it is difficult and awkward to use, as a pressure has to be applied on the piston rod when the cannula is entered into the tissue. When this pressure is reduced, aspiration will be achieved. Moreover, it is difficult for anyone to determine the quantity of aspiration with such a syringe.
A hypodermic syringe is also known where the aspiration can be achieved without applying any pre-pressure on the actuating member. This hypodermic syringe, however, shows a very complicated and expensive design and it is not appropriate for non-recurrent use. According to another known proposal the standard piston is used also for aspiration, but this embodiment leads to the complication that the friction between the piston and the inner side of the cylinder has to be less than the friction between the piston rod and the inner side of the cylinder, to be able to perform the aspiration. This, however, results in the fact that the double friction at injection may be troublesome.